Catalyst structures

ABSTRACT

A catalyst structure suitable for use in an ammonia oxidation process is described including a plurality of shaped catalyst units supported on one or more members in a spaced relationship that allows the structure to flex.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase application of PCTInternational Application No. PCT/GB2011/050047, filed Jan. 13, 2011,and claims priority of British Patent Application No. 1002378.6, filedFeb. 12, 2010, the disclosures of both of which are incorporated hereinby reference in their entireties for all purposes.

FIELD OF THE INVENTION

This invention relates to catalyst structures used in fixed bedarrangements and in particular catalyst and fixed bed arrangements usedin ammonia oxidation processes.

BACKGROUND OF THE INVENTION

Ammonia oxidation processes to produce nitric acid and hydrogen cyanideusing precious metal gauze catalysts are well established. In themanufacture of nitric acid, ammonia is oxidised with air to nitricoxide, while in the manufacture of hydrogen cyanide a mixture of ammoniaand methane (often as natural gas) is oxidised with air. Both aretypically performed by contacting the gases with a precious metalcatalyst often in the form of a gauze prepared from platinum or aplatinum-alloy. In both processes, the gas mixture is passed at anelevated temperature (e.g. 800 to 1000° C.) over a catalyst to effectthe oxidation.

Recently, improvements in base-metal catalysts have offered alternativesto precious metals with the added benefit of producing low levels ofnitrous oxide, which is a potent greenhouse gas.

Particulate ammonia oxidation catalysts based on cobalt mixed metaloxides, such as those described in WO98/28073, have proven capable ofperforming this task with excellent efficiency and the desiredselectivity. The catalytic oxidation of ammonia is very fast, so theparticulate beds are typically less than 500 mm thick. In such beds,maintaining a uniform distribution of pellets and hence of gas flowthrough the bed can be difficult. This arises from various factors,including plant vibration and variable thermal profiles across thecatalyst, but predominantly is due to the effect of the substantialchange in diameter of the reaction vessels, as they change temperaturefrom ambient to operating conditions above 850° C. and then back againon shut down. The frequency of such shut downs, can be relatively high,with a consequent effect arising, potentially cumulatively, on eachcycle. The effect of severe bed thinning can occur in particular aroundthe periphery of the catalyst bed where the resulting bypass of ammoniacan reduce process efficiency below an economic level, as well asincreasing emissions of greenhouse gases and, in severe cases, producean explosion hazard.

This problem has been successfully solved using special catalyst supportbaskets for instance as described in WO03/011448. These counteract thedetrimental effects of expansion and contraction of the catalyst bedstructure. They are however, complex to fabricate and need to becarefully sealed within the reaction vessel to avoid gaps that canthemselves promote gas bypass.

A stable, thin bed of pelletised catalyst, capable of achieving the highselectivity for the desired oxidation product, without either excessivepressure drop across it or bypass around or through it, is thereforeextremely desirable.

Moreover, with metal oxide-based ammonia oxidation catalysts it has beenfound that careful attention to the start-up process, also known aslight-off, is required to ensure the catalytic reaction is establishedat or very close to the top of the bed to reduce risk of quenchingresulting from the relatively low thermal conductivity of the catalystcompared with precious metal gauzes. A similar quenching effect may alsobe observed where the feed gases contain appreciable amounts of sulphurcompounds, which can poison cobalt-based catalysts. While this may besolved in some circumstances using hybrid arrangements of precious metalgauzes in combination with particulate ammonia oxidation and/or nitrousoxide abatement catalysts as described for example in WO04/096703 andWO04/096702, there remains a need to improve the metal oxide catalystlight-off ability and resistance to poisoning.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a catalyst structuresuitable for use in an ammonia oxidation process comprising a pluralityof shaped catalyst units supported on one or more members in a spacedrelationship that allows the structure to flex.

The invention further provides the use of the flexible catalyststructure in an ammonia oxidation process.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by reference to the drawings inwhich:

FIG. 1 depicts a linear catalyst structure according to an embodiment ofthe present invention in which a plurality of shaped catalyst units aresupported on a wire, which is fed through a through-hole in each of thepellets,

FIG. 2 depicts a cross-section through one of the shaped catalyst unitsin the embodiment of FIG. 1,

FIG. 3 depicts the embodiment of FIG. 1 coiled and disposed on a steelmesh,

FIG. 4 depicts a fixed bed comprising the embodiment in a coiledstructure supported on a steel mesh of FIG. 3 in combination with aloose particulate nitrous oxide decomposition catalyst, and

FIG. 5 depicts a mat structure formed from a plurality of linearstructures according to the embodiment of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The supported shaped catalyst units of the present invention may bepellets, spheres, rings, cylinders, extrudates, and the like, which maybe single- or multi-holed and lobed or fluted. Pelleted catalysts arepreferred as they have higher strength. The shaped units typically havemaximum and minimum dimensions, i.e. width and length, in the range 1.5to 20 mm, particularly 3 to 10 mm. The aspect ratio of the shaped units,i.e. the ratio of the maximum to minimum dimensions, is preferably lessthan 3. Two or more different catalyst particle sizes may advantageouslybe used to control the flow of gases through the catalyst bed or alterthe flexibility of the catalyst structure. These properties may also becontrolled using lobed or fluted catalyst shapes that may be designed tointer-lock. Most desirably, the supported shaped units have one or more,e.g. up to 10, through-holes arranged symmetrically to maximise thestrength. The through holes increase the geometric surface area of thecatalyst and also allow the catalyst units to be “strung” on thesupporting member or members. The catalyst units may be free to movealong the elongate support member or may be fixed in place using aninert cement or other means. Cylindrical pellets having 1-5 throughholes and optionally 3-5 lobes or flutes running along the length of theexterior of the cylinder are most preferred.

The invention may be applied to any particulate catalyst. However,preferably the particulate catalyst is an ammonia oxidation catalyst, anitrous oxide decomposition catalyst, or a mixture thereof. In addition,particulate inert materials, such as alpha alumina or other inertrefractory oxide, may be included in the structure.

In one embodiment, the particulate catalyst is an ammonia oxidationcatalyst. The ammonia oxidation catalyst may comprise a platinum groupmetal (PGM) catalyst, such as a Rh- and/or Ir-based catalyst, or maycomprise a base metal or base metal oxide, especially where the basemetal is a transition metal or a rare earth metal, and may, for example,comprise one or more of iron, nickel, copper, cobalt, manganese, silver,or supported platinum, palladium or ruthenium. The catalyst may alsocomprise a mixture of one or more base metals with one or more preciousmetals. Hence, ammonia oxidation catalysts suitable for use in thepresent invention include cobalt-containing and non-cobalt-containingammonia oxidation catalysts and mixtures of these. Included within theseare supported PGM catalysts, La₂O₃, Co₃O₄ optionally with minorquantities of Li₂O, spinels, such as CoAl₂O₄, substituted ABO₃materials, perovskites, such as LaCoO₃, including LaCoO₃ in whichpartial substitution (e.g. up to 20 mole%) of the A-site has been madeby e.g. Sr or Ce, or partial substitution (e.g. up to 50 mole %) of theB-site has been made by e.g. Cu, La₂CoO₄, CO₃O₄ supported on alumina,thoria, ceria, zinc oxide or calcium oxide, Co₃O₄, or Bi₂O₃ promoted byrare earth elements or thorium, and optionally containing one or more ofoxides of Mn, Fe, Mg, Cr or Nb, CoO_(x) with Pt on a support.Particularly suitable catalyst supports include refractory oxides suchas alumina, titania and zirconia and mixtures thereof.

In another embodiment the catalyst structure comprises a particulatenitrous oxide decomposition catalyst. Preferably, a particulate nitrousoxide decomposition catalyst is provided in the reactor at the ammoniaoxidation stage to decompose the nitrous oxide by conversion of thenitrous oxide to either (a) nitrogen by catalytic reduction or (b)nitric oxide by catalytic oxidation according to the followingequations;

N₂O→2N₂+½O₂   (a)

2N₂O+O₂→4NO.   (b)

The nitrous oxide decomposition catalyst may be a supported metal, apure or mixed metal oxide or a zeolitic system (for example thosedescribed on pages 30-32 of Kapteijn et al., Applied Catalysis B:Environmental, 9 (1996) pages 25-64 and the references providedtherein). Supported metal nitrous oxide abatement catalysts that may beused in the present invention include one or more of rhodium, ruthenium,palladium, chromium, cobalt, nickel, iron, and copper on shaped units ofoxides of alkaline earth metals, e.g. magnesium (Mg) or calcium (Ca),alumina, silica, titania, or zirconia. The metal loading in thesupported metal nitrous oxide decomposition catalysts will depend uponthe activity of the metal and the nature of the support used. The metalloading may be 1% by weight or less, but may be greater than 20% byweight. The supported metal catalyst may form oxide phases on thesupport under the reaction conditions. Hence, suitable nitrous oxidedecomposition catalysts include oxides of rhodium (Rh), iridium (Ir),cobalt (Co), iron (Fe), nickel (Ni), copper Cu(II), lanthanum (La),calcium (Ca), strontium (Sr), vanadium V(III), hafnium (Hf), manganeseMn(III), cerium (Ce), thorium (Th), tin (Sn), chromium (Cr), magnesium(Mg), zinc (Zn), and cadmium (Cd), preferably, Rh, Ir, Co, Fe and Ni.Supported metal oxides that may be used in the present invention includeany of the above pure oxides, particularly oxides of Fe, Cr(III),Mn(III), Rh, Cu and Co supported on oxides of alkaline earth metals e.g.magnesium or calcium, alumina, silica titania, zirconia or ceria.Preferably the supported oxide comprises between 0.5 and 50% by weightof the pure metal oxide catalyst.

Mixed metal oxides effective as nitrous oxide decomposition catalystsinclude doped-oxides or solid solutions, spinels, pyrochlores, andperovskites. Other useful mixed oxide catalysts that may be used in theprocess of the present invention include transition metal-modifiedhydrotalcite structures containing Co, Ni, Cu, La, Mg, Pd, Rh, and Ruand solid solutions comprising Co(II) oxide and Mn(III) oxide inmagnesia or alumina. However preferred mixed oxide nitrous oxidedecomposition catalysts are spinels and perovskites. Spinel catalyststhat may be used in the present invention may be of formula M¹M²O₄wherein M¹ is selected from Co, Cu, Ni, Mg, Zn, and Ca, M² is selectedfrom Al, Cr, or Co (and thus also includes Co₃O₄), Cu_(x)CO_(3-x)O₄(where x=0−1), Co_(x′)Mg_(1-x′)Al₂O₄ (where x′=0−1), Co_(3″)Fe_(x″)O₄,or CO_(3-x″)Al_(x″)O₄ (where x″=0−2). A preferred nitrous oxideabatement catalyst is described in WO 02/02230. The catalyst comprises0.1-10 mol % Co_(3-x)M_(x)O₄, where M is Fe or Al and x=0−2, on a ceriumoxide support. The catalyst may also contain 0.01-2 weight % ZrO₂. Asuitable non-Co containing spinel catalyst is CuAl₂O₄.

Perovskite nitrous oxide decomposition catalysts may be represented bythe general formula ABO₃ wherein A may be selected from La, Nd, Sm andPr, B may be selected from Co, Ni, Cr, Mn, Cu, Fe, and Y. Partialsubstitution of the A-site (e.g. up to 20 mol %) may be performed withdivalent or tetravalent cations, e.g. Sr²⁺ or Ce⁴⁺, to provide furtheruseful nitrous oxide decomposition catalysts. In addition, if desired,partial substitution of one B-site element (e.g. up to 50 mol %) withanother may be performed to provide further useful nitrous oxidedecomposition catalysts. Suitable perovskite catalysts include LaCoO₃,La_(1-x)Sr_(x)CoO₃, La_(1-x)Ce_(x)CoO₃ (where x≦0.2), andLaCu_(y)co_(1-y)O₃ (where y≦0.5).

Preferred nitrous oxide decomposition catalysts are supported Rhcatalysts and supported or unsupported pure and mixed metal oxides ofone or more of Co, Mn, Fe, Cu, Cr, and Ni, preferably Co in a spinel orperovskite structure.

In a preferred embodiment, the nitrous oxide decomposition catalyst isalso an effective ammonia oxidation catalyst. Accordingly, the use of acatalyst that acts both as an ammonia oxidation catalyst and as anitrous oxide decomposition catalyst offers practical advantages incatalyst assembly design and construction. Hence a particularlypreferred catalyst is a particulate composition containing oxides ofcobalt and other metals, particularly rare earths, for example asdescribed in EP-B-0946290. The preferred catalyst comprises oxides of(a) at least one element Vv selected from cerium and praseodymium and atleast one element Vn selected from non-variable valency rare earths andyttrium, and (b) cobalt, said cobalt and elements Vv and Vn being insuch proportions that the (element Vv plus element Vn) to cobalt atomicratio is in the range 0.8 to 1.2, at least some of said oxides beingpresent as a mixed oxide phase with less than 30% of the cobalt (byatoms) being present as free cobalt oxides. Preferably less than 25% (byatoms) of the cobalt is present as free cobalt oxides, and in particularit is preferred that less than 15% (by atoms) of the cobalt is presentas the cobalt monoxide, CoO. The proportion of the various phases may bedetermined by X-ray diffraction (XRD) or by thermogravimetric analysis(TGA) making use, in the latter case, of the weight loss associated withthe characteristic thermal decomposition of Co₃O₄ which occurs atapproximately 930° C. in air. Preferably less than 10%, particularlyless than 5%, by weight of the composition is free cobalto-cobalticoxide and less than 2% by weight is free cobalt monoxide.

Thus, there may be a Perovskite phase, e.g. VnCoO₃ or VvCoO₃, mixed withother phases such as Vv₂O₃, Vn₂O₃, (Vv_(x)Vn_(1-x))₂O₃, orVvxVn_(1-x)O₂. A particularly preferred catalyst is a La_(1-x)Ce_(x)CoO₃material. Such catalysts may be prepared according to examples 2 and 3of EP-B-0946290, which is incorporated herein by reference.

The shaped catalyst units are supported on one or more members in aspaced relationship that allows the structure to flex.

It is preferred that the shaped catalyst units are supported on one ormore elongate supporting members, which are suitably flexible, such thatthe structure may flex. Suitable flexible members are metal or ceramicwires, which may be wound into cables of suitable diameter for use ascatalyst supports. The catalyst units may be conveniently supportedinternally by “stringing” them onto the supporting member in a spacedrelationship, to form a linear structure, e.g. akin to a string ofpearls, in which the support member passes through the catalyst units.Support members may run both externally and internally. For example,with single holed-pellets, a steel support wire may run internallythrough the holes in the pellets and, if desired in addition, one ormore precious metal wires may be wound around the exterior surfaces ofthe pellets, e.g. in a groove or grooves along their exterior surfaces.Furthermore, two or more support members may pass through adjacent holesin multi-holed catalyst units to increase strength or allow fabricationof more complex structures.

Where elongate catalyst shapes are used, the catalyst units arepreferably supported on the support member with the support co-axial tothe longest dimension. Thus cylindrical catalyst units are preferablysupported along their length with the ends of adjacent units facing eachother.

By the term “in a spaced relationship” we mean that adjacent catalystshaped units are separated by a distance that allows the catalyststructure to flex, i.e. that allows it to bend. The separation ofadjacent shaped units depends on the sizes of the shaped units but maybe in the range 1-10 mm. Where the catalyst units are in close proximityto each other, e.g. where the separation of adjacent units is less thatthe unit diameter, it is possible to use catalyst shapes that allow thestructure to flex, such as by chamfering the edges of cylindrical shapedunits or using domed shapes, including spheres. In this way the risk ofdamage to the catalyst units and the formation of fines/dust may bereduced.

The linear structures or “catalyst strings” may be used in a number ofways in the reaction vessel to overcome the problems of the existingcatalyst beds.

Two or more of the linear catalyst structures may be connected to eachother to form a sheet or mat structure and two or more mat structuresmay be arranged in layers to form three-dimensional ‘bed’ structures.The same or different catalysts may be combined in various arrangementsusing combinations of linear structures having different properties. Inthis way mixed catalyst structures may be used with new and improvedperformance, but which are more readily separated than loose particulatemixed beds.

In the sheet structure, the support members may be aligned such thatthey are parallel or perpendicular to each other, which may offeradditional strength. If desired the linear structures may also beinterwoven to further increase the strength of the catalyst structure.Alternatively, or additionally, the support members in the sheet and bedstructures may be connected to each other by connectors, such as wirestaples or the like, to further strengthen the structure.

The support members, and connectors if used, may be suitably made fromthe high-temperature-stable steels used currently in the fabrication ofthe catalyst bed equipment. Alternatively or in addition, for ammoniaoxidation catalysts, one or more precious metal or precious metal alloysmay be present in the support member. Using metal supports andconnectors improves the heat transfer within the catalyst structure. Thecatalyst structure may be designed to leave exposed sections of metallicsupport member, which will heat up quickly and transfer heat to thesupported shaped catalyst units. In ammonia oxidation, this may improvethe catalyst light off and activity/selectivity at start-up. Inparticular, a more rapid light off may be achieved by supporting atleast some of the shaped catalyst units on a support member, either inthe form of a wire or cable, which comprises a platinum alloy. The useof platinum alloy supports and connectors may also improve resistance topoisoning, particularly by sulphur. Hence using support members andconnectors comprising platinum or a platinum alloy offers thepossibility of increasing the catalyst activity. Using a support membercomprising palladium or a palladium alloy may also allow the catalyststructure to capture platinum from upstream platinum gauzes, if used.

Whereas the linear structures or mats of catalyst pellets may bedirectly deployable in a reactor for controlling bypass in existingloose catalyst fixed bed configurations of catalyst, for thin bedcatalysis, they are desirably fixed using connectors, such as wirestaples, to a support mesh of sufficient strength to enable easyinstallation and stability to allow removal at the end of service of thecatalyst. A suitable mesh may be made of steel, platinum, a platinumalloy, or a palladium alloy and may comprise a single layer or aplurality of layers.

The staples or wires used to fix the linear structures or mats to thesupport mesh may also be a catalytically active platinum alloy toimprove ease of light off and resistance to poisoning.

The linear structures may be deployed as coils, either tightly wound orwith spacing defined by other functionality. One key attribute of thisdesign is that the support mesh may be installed into the reactor in thesame fashion as would a knitted or woven platinum gauze pack such as iscommonly used for ammonia oxidation. The thermal expansion of the meshmay then follow that of the reaction vessel, and as a result of thefixtures would ensure the linear structures of catalyst units alsofollow the various expansion and contraction cycles of plant start up,service and shut down, ensuring even spacing and distribution of thecatalyst for the whole campaign, and providing a basis for eliminationof incomplete reaction resulting from gas bypass or streaming.Alternatively, two- and three-dimensional mats of catalyst pellets maybe fixed to the interior wall of the reactor or catalyst bed supportingstructure, ensuring the same simple means of maintaining catalystdensity at the periphery of the bed. Additional certainty of avoidanceof bypass may be given by incorporating a rim of solid metal, fixed tothe inside of the reaction vessel or catalyst bed support structure andextending in towards the center.

The catalyst structures of the present invention also offer the abilityto more readily recover the catalyst units from the reactor and, ifdifferent catalysts have been used, separate them for metal recoveryand/or recycling.

Furthermore, using a catalytically active supporting mesh for thecatalyst structures may provide a “fail safe” catalyst arrangement inammonia oxidation reactors. Once the thickness of the particulatecatalyst bed has been increased beyond the minimum required for fullcompletion of the desired chemical reaction, the underlying metalliccatalyst will take little or no part in the primary reaction and so willbe unaffected. Normally in the case of ammonia oxidation using platinumgroup metal alloy gauzes, the gauzes actively catalysing the reactionare severely re-structured, weakening them, causing them to lose weightand giving them limited life. If these, or similar alloys, were presentdownstream of the particulate catalyst as part or the whole of thesupport mesh, they would not normally be catalytically active and wouldtherefore not degrade. Therefore until they were required for “emergencycatalysis” to avoid the potentially dangerous passing of unreactedammonia in the downstream part of the chemical plant, they would remainviable and strong. Operating costs may be reduced substantially as aresult. This system thus offers an emergency failsafe situation suitablefor optimum plant operation, such as might be required in the event ofdramatic and complete poisoning/degradation of the whole bed ofparticulate ammonia oxidation catalyst.

The invention further includes a fixed catalyst bed comprising one ormore catalyst structures comprising a plurality of shaped catalyst unitssupported on one or more members in a spaced relationship that allowsthe structure to flex.

In a fixed bed, the catalyst strings, which may be constrained on a meshor mesh pack may be covered with multiple layers of strings either meshconstrained or not, or two or three dimensional mats or loose pellets.In this latter scenario, the pellet size may be selected to match apredefined spacing of the constrained string- or mat-structure, suchthat the spacing limits movement of the loose pellets to control andmaintain the density of the pellet bed across the whole catalyst area,and so maintain even gas flow and optimised catalytic behaviour acrossthe bed. Thus the fixed bed may comprise one or more of the linearcatalyst structures and a plurality of loose catalyst pellets. The loosecatalyst pellets may be the same or different in composition and/orshape and/or size to the supported shaped catalyst units. The loosecatalyst pellets may be solid or have one or more through-holes andtypically have maximum and minimum dimensions, i.e. width and length, inthe range 1.5 to 20 mm, particularly 3 to 10 mm. The aspect ratio of theloose catalyst pellets, i.e. the ratio of the maximum to minimumdimensions, is preferably less than 3. The design of the fixed bed canbe utilised to maximise catalytic reaction with minimum pressure drop ofgas as it passes through the bed. In one embodiment solid cylindricalcatalyst pellets with a diameter in the range 0.5-2D, where D is thediameter of the supported shaped catalyst unit, are combined with thecatalyst structure to form a catalyst bed.

It may be possible to construct thinner beds than those typicallyemployed for loose particulate catalysts alone using the catalyststructure of the present invention. Thin beds (<300 mm deep, preferably<150 mm deep) may offer reduced pressure drop and may be easier toinstall in existing ammonia oxidation reactors.

The deployment of linear structures or mats of supported particulatecatalyst has a further major advantage over loose beds of catalystpellets. These latter depend on the underlying support grid to maintainthem in position. However these are subject to warping that can promotethinning of the catalyst beds. They are also susceptible to separationof the individual strands of wire making them up, caused by mechanicaland thermo-mechanical effects. This can in extreme cases cause local,but potentially extensive formation of holes through which the loosepellets can be lost into the bowels of the reaction chamber. This isundesirable both from a contamination perspective and the fact that thecatalytic pellets may be capable of promoting undesirable chemicalreaction down stream of the primary reaction zone. Local loss ofcatalyst could also promote gas streaming, i.e. bypass of the catalystbed, with significant loss of reaction efficiency. The use of string ormats in regions particularly prone to structural change of the supportcan reduce this problem significantly; both maintaining conversionefficiency and relative pack longevity. This same benefits, inparticular the ability of the bed to expand and contract withoutperipheral thinning, but even more clearly defined, arises where thewhole bed consists of the linear and/or mat catalyst structures.

Whereas the catalyst structure may be used in a reaction vessel withparticulate ammonia oxidation and/or nitrous oxide decompositioncatalyst only, it is preferred that the particulate catalyst is used incombination with a precious metal ammonia oxidation catalyst.Accordingly, the invention includes a catalyst combination comprising aprecious metal ammonia oxidation catalyst gauze and a catalyst structurecomprising a plurality of shaped ammonia oxidation catalyst particlesand/or nitrous oxide decomposition catalyst particles supported on oneor more members in a spaced relationship that allows the structure toflex.

Precious metal gauzes may be formed by weaving or knitting or otherwiseforming precious metal filaments into a gauze-like structure. Suchcatalyst gauzes are well established and may consist of platinum orplatinum alloy filaments of thickness from 0.02 to 0.15 mm woven toprovide rectangular interstices, knitted to provide a regular loopedstructure or simply agglomerated to provide a non-woven irregularstructure. Herein the term “filament” is meant to include wires thathave a substantially circular cross-section and also wires that areflattened or otherwise shaped and thereby have a non-circular crosssection. Woven gauzes are well established and typically comprise 0.076mm diameter wire, woven to provide 1024 apertures per square centimeterand prepared to a specific weight per unit area dependant upon the wirecomposition. Knitted gauzes offer a number of advantages in terms ofcatalyst physical properties, catalyst activity and lifetime. Knittedgauzes comprise a regular looped structure and may be formed using wirewith diameters in the same range as woven materials, in a variety ofshapes and thicknesses using variety of stitches such as tricot,jacquard, satin stitch (smooth sunk loops) and raschel. EP-B-0364153,page 3, line 5 to line 56 describes knitted gauzes of particular use inthe present invention. Non-woven gauzes are described for example in GB2064975 and GB 2096484.

The precious metal ammonia oxidation catalyst is preferably platinum(Pt) or a platinum alloy, such as an alloy of platinum with rhodium (Rh)and/or palladium (Pd). Such alloys may contain ≧50% preferably ≧85% Ptby weight. For example, alloys often used in ammonia oxidation in theproduction of nitric acid or hydrogen cyanide include 10% Rh 90% Pt, 8%Rh 92% Pt, 5% Pd 5% Rh 90% Pt, and 5% Rh 95% Pt. Alloys containing up toabout 5% of iridium (Ir) may also be used in the present invention. Theprecious metal catalyst may desirably be formulated to reduce nitrousoxide by-product formation, and may thus have an increased rhodium (Rh)or palladium (Pd) content, or may contain other components, such ascobalt (Co). In particular high Pd alloys, comprising 35-45% Pd, 65-55%wt Pt, and 0-5% wt Rh by weight may be used in at least part of thegauze pack to provide a stable, low N₂O catalyst arrangement.

In a conventional nitric acid plant, the number of gauzes employeddepends on the pressure at which the process is operated. For example ina plant operating at low pressure, e.g. up to about 5 bar abs.,typically <10, often 3 to 6 gauzes may be employed, while at higherpressures, e.g. up to 20 bar abs., a greater number of gauzes,typically >20, often 35-45, may be employed. The gauzes, which arenormally circular, may be incorporated individually into the reactor ormay be pre-formed into a pad comprising a number of gauzes that may bewelded at their periphery. The pad may comprise a combination of wovenand knitted or possibly non-woven gauzes whose elemental composition maybe the same or different. If adjacent woven gauzes are present, tofacilitate replacement, they are preferably arranged so that their warpsor wefts are at an angle of 45° to each other. Angular displacement,suitably at 90°, may also be used between adjacent woven gauzes toreduce opportunities for gas channelling.

Catchment gauzes based on palladium are also desirably used in ammoniaoxidation plants to act as so-called “getters” or collectors of‘vaporised’ platinum lost by chemical action, evaporation, or mechanicallosses from the precious metal catalyst. Such catchment gauzes may be inthe form of woven or knitted gauzes or agglomerated non-woven gauzesakin to those described above for the precious metal catalysts. Anypalladium present in a gauze will be able to catch vaporized platinumpassing over it, hence the palladium content of the catchment gauze maybe from 10 to ≧95% wt, preferably >50%, more preferably >70%. One ormore palladium based catchment gauzes may be used. The catchment gauzesmay be provided underneath the precious metal catalyst gauzesindividually or form a lower or final gauze as part of a precious metalcatalyst pad. The catchment gauzes may be knitted, e.g. according to theaforesaid EP-B-0364153 and may form a layer or layers in a preciousmetal catalyst knitted structure, e.g. a layer in a knitted pad.Alternatively, the palladium-based guard material is woven or knittedinto a precious metal ammonia oxidation catalyst gauze by using it as afilament in the weaving or knitting process. Palladium-based guardmaterials suitable for weaving or knitting into gauze structures arepalladium or palladium alloys with nickel (Ni), cobalt (Co) or gold(Au). For example a catchment gauze may be fabricated from a 95:5% wtPd:Ni alloy. In addition the palladium-based guard material maydesirably be formulated to reduce nitrous oxide by-product formation,and may thus preferably contain a small amount, e.g. <5% rhodium (Rh).In particular, palladium gauzes containing amounts of platinum andrhodium may be used. Such gauzes may comprise, for example >92% wtpalladium, 2-4% wt rhodium and the remainder platinum, or alternativelycomprise 82-83% wt palladium, 2.5-3.5% wt rhodium and the remainderplatinum. Ceramic fibers comprising an inert refractory material, suchas alumina, zirconia or the like, may also be woven or knitted intocatchment gauzes in addition to the palladium-based materials.

The supporting framework for the gauzes may be any currently in use andincludes simple girder support arrangements that extend across thevessel and so-called “baskets” in which the precious metal gauzes aresupported on the base of a cylindrical unit suspended within the ammoniaoxidation vessel.

The invention further provides an ammonia oxidation process comprisingthe step of passing a gas mixture comprising ammonia, an oxygencontaining gas, such as air, and optionally, a methane containing gasthrough the catalyst structure described herein that comprises anammonia oxidation catalyst and/or a nitrous oxide decompositioncatalyst.

In the oxidation of ammonia to nitric oxide for the manufacture ofnitric acid, the oxidation process may be operated at temperatures of750-1000° C., particularly 850-950° C., pressures of 1 (low pressure) to15 (high pressure) bar abs., with ammonia in air concentrations of7-13%, often about 10%, by volume. In the oxidation of ammonia with airin the presence of methane for the manufacture of hydrogen cyanide, theAndrussow Process, the operating conditions are similar although theoperating temperatures may be up to 1100-1150° C. The present inventionis particularly suited to processes and ammonia oxidation reactorsoperated at pressures in the range 6-15 bar abs, particularly 7-15 bar g(so-called high pressure plants) because the containment unit mayreadily be placed in the vessel without having to move or adjust heatrecovery means commonly found just below the gauzes in medium pressureand atmospheric plants.

The use of linear structures or mats of catalyst only in localisedpositions with a catalyst bed is also useful in nitrous oxide abatementcatalyst beds, where gas flow channelling and escape of loose pelletspotentially present serious performance losses. The supported catalystpellets may be the same as those used in an ammonia oxidation bed orthey may differ in composition, size, and shape or any combination ofthese. Using a nitrous oxide decomposition catalyst, the process of thepresent invention may provide aggregate N₂O levels below 1600 ppm,preferably below 600 ppm, more preferably below 500 ppm and mostpreferably below 200 ppm.

In one embodiment, the catalyst arrangement in the vessel comprises aplurality of flexible linear and/or mat structures of ammonia oxidationcatalyst, optionally with loose particulate ammonia oxidation catalystdisposed thereon, on a support mesh comprising a platinum alloy gauzepack, under which is a flexible nitrous oxide abatement catalyststructure supported on a further support mesh which may be of steel, aplatinum alloy, or a palladium catchment gauze. Preferably theparticulate ammonia oxidation and nitrous oxide decomposition catalystin the flexible structures is the same and comprises shaped units ofmixed metal rare-earth cobalt perovskite catalyst, as described inEP-B-0946290.

Alternatively, hybrid catalyst arrangements comprising a precious metalgauze on top of a particulate ammonia oxidation or nitrous oxideabatement catalyst, for example as described in the aforesaidWO04/096703 and WO04/096702, may be used in which the loose particulatecatalysts are replaced with catalyst structures according to the presentinvention.

This invention offers solutions to a number of problems including;

-   -   1. How to stabilise a thin bed of particulate catalyst against        the potentially detrimental effects of thermal and mechanical        processes that are commonly encountered in commercial ammonia        oxidation plants, while achieving a commercially viable reactant        conversion efficiency.    -   2. How to control particulate catalyst migration.    -   3. How to avoid development of variable catalyst distribution        density.    -   4. The potential to reduce normally inherent problems of        light-off common to non metallic ammonia oxidation catalysts.    -   5. The potential to provide thinner catalyst beds with reduced        pressure drop.    -   6. How to provide a fail-safe ammonia oxidation catalyst        arrangement.    -   7. How to minimize particulate catalyst loss in the event of        support mesh failure.    -   8. How to maintain catalyst retention at the edges of currently        available N2O abatement catalyst beds.    -   9. How to simply recover and separate mixed catalysts from the        reactor for metal recovery and/or recycling.

In FIG. 1, a linear catalyst structure 10 comprises a plurality ofammonia oxidation catalyst pellets 12 of a cobalt-based ammoniaoxidation catalyst strung on a metal wire 14 formed from a steel alloy.The ammonia oxidation catalyst, which also functions as a nitrous oxidedecomposition catalyst, is a Co perovskite prepared according toexamples 2 and 3 of EP-B-0946290. The pellets have a diameter of about10 mm. The catalysts in this embodiment are free to move along the wireso that they may separate as the wire is coiled.

In FIG. 2, the cross section of the pellet 12 is in the form of asymmetrical 4-hole and lobed cylinder. The holes 13 are arranged in asquare pattern, and the lobes are formed by four co-axial channels 15 inthe wall of the cylinder wall equidistant between adjacent holes. Onlyone support wire 14 passing through one hole 13 is depicted, but one ormore wires may be strung through each through-hole.

In FIG. 3, the linear structure comprising the plurality of pellets 12on support wire 14 is coiled and disposed on a steel mesh 16 to which itmay be fixed by means of wire staples (not shown).

In FIG. 4, loose cylindrical pellets 18 of a nitrous oxide abatementcatalyst are combined with the mesh-supported coiled structure of FIG. 3to form a fixed bed arrangement. A dashed line is superimposed on thefigure to indicate the position of the coiled catalyst structure withinthe bed. Wire staples 20 have been used to fix the coiled structure inplace on the support mesh.

In FIG. 5, a plurality of linear structures 10 of FIG. 1 have beenaligned with parallel support wires 14 and 14′ to form a mat structure22. The catalyst pellets 12 are arranged in an interlockingbrick-pattern by passing support wires 14 and 14′ through two of thefour holes 13 in the catalyst pellets. Dashed lines are superimposed onFIG. 5 to indicate the position of the support wires 14 and 14′.

1. A catalyst structure suitable for use in an ammonia oxidationprocess, comprising a plurality of shaped catalyst units supported onone or more members in a spaced relationship that allows the structureto flex, wherein the shaped catalyst units are pellets or extrudateswith maximum and minimum dimensions in the range 1.5 to 20 mm, and anaspect ratio less than 3, the supporting members are metal or ceramicwires, which may be wound into cables, and the separation of adjacentshaped catalyst units is in the range 1-10 mm.
 2. A catalyst structureaccording to claim 1 wherein the shaped catalyst units comprise anammonia oxidation catalyst, a nitrous oxide decomposition catalyst or amixture thereof.
 3. A catalyst structure according to claim 1 wherein aparticulate inert material is included in the structure.
 4. (canceled)5. A catalyst structure according to claim 1 wherein two or moredifferent catalyst particle sizes are present
 6. A catalyst structureaccording to claim 1 wherein the shaped catalyst units have 1 to 10through-holes.
 7. A catalyst structure according to claim 1 wherein theshaped catalyst units are domed.
 8. A catalyst structure according toclaim 1 wherein the catalyst units are domed cylindrical pellets having1-5 through holes.
 9. (canceled)
 10. A catalyst structure according toclaim 1 wherein the supporting members comprise steel or a platinumalloy.
 11. A catalyst structure according to claim 1 wherein the supportmembers run both externally and internally with regard to the shapedcatalyst units.
 12. (canceled)
 13. A catalyst structure according toclaim 1 wherein two or more linear catalyst structures, in which thecatalyst units are the same or different, are connected to each other toform a mat structure
 14. A catalyst structure according to claim 13wherein two or more mat structures are arranged in layers to form athree-dimensional bed structure.
 15. A catalyst structure according toclaim 13 wherein the support members in the mat are connected to eachother by connectors.
 16. A catalyst structure according to claim 1disposed on a support mesh.
 17. A catalyst structure according to claim16 wherein the support mesh is made of steel, platinum, a platinum alloyor a palladium alloy, and comprises a single layer or a plurality oflayers.
 18. A catalyst structure according to claim 16 wherein thecatalyst structure is fixed using connectors to the support mesh.
 19. Acatalyst structure according to claim 16 further comprising loosecatalyst pellets.
 20. A catalyst structure according to claim 1 furthercomprising one or more precious metal ammonia oxidation catalyst gauzes.21. An ammonia oxidation process comprising the step of passing a gasmixture comprising ammonia, an oxygen containing gas and optionally amethane containing gas through a catalyst structure according to claim 1comprising an ammonia oxidation catalyst and/or a nitrous oxidedecomposition catalyst.